1. Field of the Invention
The present invention relates to an illumination optical system for illuminating a surface to be illuminated, and further, relates to an image display apparatus (a liquid crystal projector or the like) in which a surface (an image display element such as a liquid crystal panel) is illuminated with the use of the illumination optical system, and the light beam from the surface to be illuminated is projected onto a projection surface such as a screen.
2. Description of the Related Art
Public attention has been focused on a projector having such a configuration that the light beam which has been modulated by a liquid crystal light valve in accordance with image data, is enlarged and projected onto a screen or the like. In this configuration of the projector, it is important to display an image projected on the screen with uniform brightness over the entire screen.
A well-known illumination optical system in the above-mentioned projector, has a configuration as shown in, for example, FIG. 8. That is, a light beam emitted from a light source 101 is turned by a parabolic surface reflector 102 into a substantially parallel light beam, and is then output from the therefrom. This parallel light beam is split and focused by a fly-eye lens (a lens array in which micro spherical lenses are two-dimensionally arranged) 103. The respective split light beams are focused in the vicinity of a second fly-eye lens 104 so as to produce an image of the light source (a two-dimensional image of the light source). The micro lenses which constitute these fly-eye lenses 103, 104 have a rectangular shape which is similar to the shape of the liquid display panel, that is, the surface to be illuminated. The split light beams output from the second fly-eye lens are focused by a condenser lens 105, and are led through a color separation optical system 106 and the like, and accordingly, the plurality of split beams illuminates the liquid crystal panel 107 in superposition. It is noted that only essential components of the illumination optical system are shown in FIG. 8 in order to facilitate the explanation of the function of the illumination optical system.
However, the above-mentioned illumination optical system, in general has a tendency such that the higher the light available efficiency, the larger the angular distribution of the light beam, and accordingly, has caused a problem of deterioration of the image quality such as irregularity or lowering of contrast when an optical component which is sensitive to an angular characteristic is used in the illumination optical system, and in particular, when a color separation film (dichroic mirror, dichroic prism or the like) or a polarization separation film (polarized film splitter or the like) inclined to the optical axis of an illumination optical system is used in a color separation optical system.
In order to prevent the above-mentioned deterioration of the image quality, there has been used an asymmetric optical system including an optical element having a small angular distribution in a direction in which it is sensitive to the angular distribution but having a large angular distribution in a direction in which it is insensitive to the angular distribution, as disclosed in Japanese Patent Laid-Open No. H06-75200 or Japanese Patent Laid-Open No. 2004-45907.
The Japanese Patent Laid-Open No. H06-75200 discloses a configuration which can reduce uneven color by using a cylindrical lens array which is a one-dimensional array serving as an optical integrator and as well utilizing Koehler illumination in the bending direction of an element which is highly sensitive to a one-dimensional angle, such as a dichroic mirror.
Further, Japanese Patent Laid-Open No. 2004-45907 discloses a configuration which can restrain the angular distribution of the light beam in one cross-sectional direction by setting a stop at a pupil position in a direction in which an angular sensitivity of a thin film component is high, thereby it is possible to improve the contrast.
However, in the Japanese Patent Laid-Open No. H06-75200, no superposing illumination is made in a cross-section (Koehler illumination cross-section) in which a cylindrical lens array has not a refractive power, uniform illumination cannot be obtained at the light valve surface. Accordingly, a relatively flat distribution should be selectively used from a nonuniform illumination distribution, resulting in low light available efficiency. Further, since the light beam from the light source to the condenser lens has a small angular distribution, although an image quality deteriorating effect by the dichroic mirror located therebetween is reduced, image quality deterioration caused by an element having a high angular sensitivity, such as a liquid crystal panel or the dichroic mirror, located after the condenser lens is inevitable as the light beam is converged by the condenser lens just before the liquid crystal panel. Further, in the cross-section in which no superposing illumination is made, when the light source causes uneven brightness due to fluctuation (arc jump, deterioration or the like) of the light source, the illumination distribution of the light valve also varies, resulting in unevenness at a projected screen.
Further, in the Japanese Patent Laid-Open No. 2004-45907, superposing illumination is made in cross-sections in both direction, although the light source can hardly have an effect, extreme lowering of the light available efficiency is inevitable since the light beam is limited by a stop. There has been disclosed such a configuration that the principle point positions of a part of lenses (an optical system between the lens array and the panel), other than the stop, are different from each other between two cross-sections in order to cause angular distributions in the two cross-sections to be different from each other. However, in a method stated in the description of the embodiments (a principal point of a collimator lens is changed), the boundary of the illumination zone at the surface of the liquid crystal display panel is unclear, resulting in lowering of brightness or uneven illumination. Further, since the telecentric condition (the exit pupil is sufficiently far from the panel surface) becomes unsatisfactory, there has been raised such a problem that uneven contrast and uneven color would be caused.